The ptrace() system call provides a means by which one process (the
"tracer") may observe and control the execution of another process
(the "tracee"), and examine and change the tracee's memory and
registers. It is primarily used to implement breakpoint debugging
and system call tracing.
A tracee first needs to be attached to the tracer. Attachment and
subsequent commands are per thread: in a multithreaded process, every
thread can be individually attached to a (potentially different)
tracer, or left not attached and thus not debugged. Therefore,
"tracee" always means "(one) thread", never "a (possibly
multithreaded) process". Ptrace commands are always sent to a
specific tracee using a call of the form
ptrace(PTRACE_foo, pid, ...)
where pid is the thread ID of the corresponding Linux thread.
(Note that in this page, a "multithreaded process" means a thread
group consisting of threads created using the clone(2)CLONE_THREAD
flag.)
A process can initiate a trace by calling fork(2) and having the
resulting child do a PTRACE_TRACEME, followed (typically) by an
execve(2). Alternatively, one process may commence tracing another
process using PTRACE_ATTACH or PTRACE_SEIZE.
While being traced, the tracee will stop each time a signal is
delivered, even if the signal is being ignored. (An exception is
SIGKILL, which has its usual effect.) The tracer will be notified at
its next call to waitpid(2) (or one of the related "wait" system
calls); that call will return a status value containing information
that indicates the cause of the stop in the tracee. While the tracee
is stopped, the tracer can use various ptrace requests to inspect and
modify the tracee. The tracer then causes the tracee to continue,
optionally ignoring the delivered signal (or even delivering a
different signal instead).
If the PTRACE_O_TRACEEXEC option is not in effect, all successful
calls to execve(2) by the traced process will cause it to be sent a
SIGTRAP signal, giving the parent a chance to gain control before the
new program begins execution.
When the tracer is finished tracing, it can cause the tracee to
continue executing in a normal, untraced mode via PTRACE_DETACH.
The value of request determines the action to be performed:
PTRACE_TRACEME
Indicate that this process is to be traced by its parent. A
process probably shouldn't make this request if its parent
isn't expecting to trace it. (pid, addr, and data are
ignored.)
The PTRACE_TRACEME request is used only by the tracee; the
remaining requests are used only by the tracer. In the
following requests, pid specifies the thread ID of the tracee
to be acted on. For requests other than PTRACE_ATTACH,
PTRACE_SEIZE, PTRACE_INTERRUPT, and PTRACE_KILL, the tracee
must be stopped.
PTRACE_PEEKTEXT, PTRACE_PEEKDATA
Read a word at the address addr in the tracee's memory,
returning the word as the result of the ptrace() call. Linux
does not have separate text and data address spaces, so these
two requests are currently equivalent. (data is ignored; but
see NOTES.)
PTRACE_PEEKUSER
Read a word at offset addr in the tracee's USER area, which
holds the registers and other information about the process
(see <sys/user.h>). The word is returned as the result of the
ptrace() call. Typically, the offset must be word-aligned,
though this might vary by architecture. See NOTES. (data is
ignored; but see NOTES.)
PTRACE_POKETEXT, PTRACE_POKEDATA
Copy the word data to the address addr in the tracee's memory.
As for PTRACE_PEEKTEXT and PTRACE_PEEKDATA, these two requests
are currently equivalent.
PTRACE_POKEUSER
Copy the word data to offset addr in the tracee's USER area.
As for PTRACE_PEEKUSER, the offset must typically be word-
aligned. In order to maintain the integrity of the kernel,
some modifications to the USER area are disallowed.
PTRACE_GETREGS, PTRACE_GETFPREGS
Copy the tracee's general-purpose or floating-point registers,
respectively, to the address data in the tracer. See
<sys/user.h> for information on the format of this data.
(addr is ignored.) Note that SPARC systems have the meaning
of data and addr reversed; that is, data is ignored and the
registers are copied to the address addr. PTRACE_GETREGS and
PTRACE_GETFPREGS are not present on all architectures.
PTRACE_GETREGSET (since Linux 2.6.34)
Read the tracee's registers. addr specifies, in an
architecture-dependent way, the type of registers to be read.
NT_PRSTATUS (with numerical value 1) usually results in
reading of general-purpose registers. If the CPU has, for
example, floating-point and/or vector registers, they can be
retrieved by setting addr to the corresponding NT_foo
constant. data points to a struct iovec, which describes the
destination buffer's location and length. On return, the
kernel modifies iov.len to indicate the actual number of bytes
returned.
PTRACE_SETREGS, PTRACE_SETFPREGS
Modify the tracee's general-purpose or floating-point
registers, respectively, from the address data in the tracer.
As for PTRACE_POKEUSER, some general-purpose register
modifications may be disallowed. (addr is ignored.) Note
that SPARC systems have the meaning of data and addr reversed;
that is, data is ignored and the registers are copied from the
address addr. PTRACE_SETREGS and PTRACE_SETFPREGS are not
present on all architectures.
PTRACE_SETREGSET (since Linux 2.6.34)
Modify the tracee's registers. The meaning of addr and data
is analogous to PTRACE_GETREGSET.
PTRACE_GETSIGINFO (since Linux 2.3.99-pre6)
Retrieve information about the signal that caused the stop.
Copy a siginfo_t structure (see sigaction(2)) from the tracee
to the address data in the tracer. (addr is ignored.)
PTRACE_SETSIGINFO (since Linux 2.3.99-pre6)
Set signal information: copy a siginfo_t structure from the
address data in the tracer to the tracee. This will affect
only signals that would normally be delivered to the tracee
and were caught by the tracer. It may be difficult to tell
these normal signals from synthetic signals generated by
ptrace() itself. (addr is ignored.)
PTRACE_PEEKSIGINFO (since Linux 3.10)
Retrieve siginfo_t structures without removing signals from a
queue. addr points to a ptrace_peeksiginfo_args structure
that specifies the ordinal position from which copying of
signals should start, and the number of signals to copy.
siginfo_t structures are copied into the buffer pointed to by
data. The return value contains the number of copied signals
(zero indicates that there is no signal corresponding to the
specified ordinal position). Within the returned siginfo
structures, the si_code field includes information (__SI_CHLD,
__SI_FAULT, etc.) that are not otherwise exposed to user
space.
struct ptrace_peeksiginfo_args {
u64 off; /* Ordinal position in queue at which
to start copying signals */
u32 flags; /* PTRACE_PEEKSIGINFO_SHARED or 0 */
s32 nr; /* Number of signals to copy */
};
Currently, there is only one flag, PTRACE_PEEKSIGINFO_SHARED, for
dumping signals from the process-wide signal queue. If this flag is
not set, signals are read from the per-thread queue of the specified
thread.
PTRACE_GETSIGMASK (since Linux 3.11)
Place a copy of the mask of blocked signals (see
sigprocmask(2)) in the buffer pointed to by data, which should
be a pointer to a buffer of type sigset_t. The addr argument
contains the size of the buffer pointed to by data (i.e.,
sizeof(sigset_t)).
PTRACE_SETSIGMASK (since Linux 3.11)
Change the mask of blocked signals (see sigprocmask(2)) to the
value specified in the buffer pointed to by data, which should
be a pointer to a buffer of type sigset_t. The addr argument
contains the size of the buffer pointed to by data (i.e.,
sizeof(sigset_t)).
PTRACE_SETOPTIONS (since Linux 2.4.6; see BUGS for caveats)
Set ptrace options from data. (addr is ignored.) data is
interpreted as a bit mask of options, which are specified by
the following flags:
PTRACE_O_EXITKILL (since Linux 3.8)
If a tracer sets this flag, a SIGKILL signal will be
sent to every tracee if the tracer exits. This option
is useful for ptrace jailers that want to ensure that
tracees can never escape the tracer's control.
PTRACE_O_TRACECLONE (since Linux 2.5.46)
Stop the tracee at the next clone(2) and automatically
start tracing the newly cloned process, which will
start with a SIGSTOP, or PTRACE_EVENT_STOP if
PTRACE_SEIZE was used. A waitpid(2) by the tracer will
return a status value such that
status>>8 == (SIGTRAP | (PTRACE_EVENT_CLONE<<8))
The PID of the new process can be retrieved with
PTRACE_GETEVENTMSG.
This option may not catch clone(2) calls in all cases.
If the tracee calls clone(2) with the CLONE_VFORK flag,
PTRACE_EVENT_VFORK will be delivered instead if
PTRACE_O_TRACEVFORK is set; otherwise if the tracee
calls clone(2) with the exit signal set to SIGCHLD,
PTRACE_EVENT_FORK will be delivered if
PTRACE_O_TRACEFORK is set.
PTRACE_O_TRACEEXEC (since Linux 2.5.46)
Stop the tracee at the next execve(2). A waitpid(2) by
the tracer will return a status value such that
status>>8 == (SIGTRAP | (PTRACE_EVENT_EXEC<<8))
If the execing thread is not a thread group leader, the
thread ID is reset to thread group leader's ID before
this stop. Since Linux 3.0, the former thread ID can
be retrieved with PTRACE_GETEVENTMSG.
PTRACE_O_TRACEEXIT (since Linux 2.5.60)
Stop the tracee at exit. A waitpid(2) by the tracer
will return a status value such that
status>>8 == (SIGTRAP | (PTRACE_EVENT_EXIT<<8))
The tracee's exit status can be retrieved with
PTRACE_GETEVENTMSG.
The tracee is stopped early during process exit, when
registers are still available, allowing the tracer to
see where the exit occurred, whereas the normal exit
notification is done after the process is finished
exiting. Even though context is available, the tracer
cannot prevent the exit from happening at this point.
PTRACE_O_TRACEFORK (since Linux 2.5.46)
Stop the tracee at the next fork(2) and automatically
start tracing the newly forked process, which will
start with a SIGSTOP, or PTRACE_EVENT_STOP if
PTRACE_SEIZE was used. A waitpid(2) by the tracer will
return a status value such that
status>>8 == (SIGTRAP | (PTRACE_EVENT_FORK<<8))
The PID of the new process can be retrieved with
PTRACE_GETEVENTMSG.
PTRACE_O_TRACESYSGOOD (since Linux 2.4.6)
When delivering system call traps, set bit 7 in the
signal number (i.e., deliver SIGTRAP|0x80). This makes
it easy for the tracer to distinguish normal traps from
those caused by a system call. (PTRACE_O_TRACESYSGOOD
may not work on all architectures.)
PTRACE_O_TRACEVFORK (since Linux 2.5.46)
Stop the tracee at the next vfork(2) and automatically
start tracing the newly vforked process, which will
start with a SIGSTOP, or PTRACE_EVENT_STOP if
PTRACE_SEIZE was used. A waitpid(2) by the tracer will
return a status value such that
status>>8 == (SIGTRAP | (PTRACE_EVENT_VFORK<<8))
The PID of the new process can be retrieved with
PTRACE_GETEVENTMSG.
PTRACE_O_TRACEVFORKDONE (since Linux 2.5.60)
Stop the tracee at the completion of the next vfork(2).
A waitpid(2) by the tracer will return a status value
such that
status>>8 == (SIGTRAP | (PTRACE_EVENT_VFORK_DONE<<8))
The PID of the new process can (since Linux 2.6.18) be
retrieved with PTRACE_GETEVENTMSG.
PTRACE_O_TRACESECCOMP (since Linux 3.5)
Stop the tracee when a seccomp(2)SECCOMP_RET_TRACE
rule is triggered. A waitpid(2) by the tracer will
return a status value such that
status>>8 == (SIGTRAP | (PTRACE_EVENT_SECCOMP<<8))
While this triggers a PTRACE_EVENT stop, it is similar
to a syscall-enter-stop, in that the tracee has not yet
entered the syscall that seccomp triggered on. The
seccomp event message data (from the SECCOMP_RET_DATA
portion of the seccomp filter rule) can be retrieved
with PTRACE_GETEVENTMSG.
PTRACE_O_SUSPEND_SECCOMP (since Linux 4.3)
Suspend the tracee's seccomp protections. This applies
regardless of mode, and can be used when the tracee has
not yet installed seccomp filters. That is, a valid
use case is to suspend a tracee's seccomp protections
before they are installed by the tracee, let the tracee
install the filters, and then clear this flag when the
filters should be resumed. Setting this option
requires that the tracer have the CAP_SYS_ADMIN
capability, not have any seccomp protections installed,
and not have PTRACE_O_SUSPEND_SECCOMP set on itself.
PTRACE_GETEVENTMSG (since Linux 2.5.46)
Retrieve a message (as an unsigned long) about the ptrace
event that just happened, placing it at the address data in
the tracer. For PTRACE_EVENT_EXIT, this is the tracee's exit
status. For PTRACE_EVENT_FORK, PTRACE_EVENT_VFORK,
PTRACE_EVENT_VFORK_DONE, and PTRACE_EVENT_CLONE, this is the
PID of the new process. For PTRACE_EVENT_SECCOMP, this is the
seccomp(2) filter's SECCOMP_RET_DATA associated with the
triggered rule. (addr is ignored.)
PTRACE_CONT
Restart the stopped tracee process. If data is nonzero, it is
interpreted as the number of a signal to be delivered to the
tracee; otherwise, no signal is delivered. Thus, for example,
the tracer can control whether a signal sent to the tracee is
delivered or not. (addr is ignored.)
PTRACE_SYSCALL, PTRACE_SINGLESTEP
Restart the stopped tracee as for PTRACE_CONT, but arrange for
the tracee to be stopped at the next entry to or exit from a
system call, or after execution of a single instruction,
respectively. (The tracee will also, as usual, be stopped
upon receipt of a signal.) From the tracer's perspective, the
tracee will appear to have been stopped by receipt of a
SIGTRAP. So, for PTRACE_SYSCALL, for example, the idea is to
inspect the arguments to the system call at the first stop,
then do another PTRACE_SYSCALL and inspect the return value of
the system call at the second stop. The data argument is
treated as for PTRACE_CONT. (addr is ignored.)
PTRACE_SYSEMU, PTRACE_SYSEMU_SINGLESTEP (since Linux 2.6.14)
For PTRACE_SYSEMU, continue and stop on entry to the next
system call, which will not be executed. For
PTRACE_SYSEMU_SINGLESTEP, do the same but also singlestep if
not a system call. This call is used by programs like User
Mode Linux that want to emulate all the tracee's system calls.
The data argument is treated as for PTRACE_CONT. The addr
argument is ignored. These requests are currently supported
only on x86.
PTRACE_LISTEN (since Linux 3.4)
Restart the stopped tracee, but prevent it from executing.
The resulting state of the tracee is similar to a process
which has been stopped by a SIGSTOP (or other stopping
signal). See the "group-stop" subsection for additional
information. PTRACE_LISTEN works only on tracees attached by
PTRACE_SEIZE.
PTRACE_KILL
Send the tracee a SIGKILL to terminate it. (addr and data are
ignored.)
This operation is deprecated; do not use it! Instead, send a
SIGKILL directly using kill(2) or tgkill(2). The problem with
PTRACE_KILL is that it requires the tracee to be in signal-
delivery-stop, otherwise it may not work (i.e., may complete
successfully but won't kill the tracee). By contrast, sending
a SIGKILL directly has no such limitation.
PTRACE_INTERRUPT (since Linux 3.4)
Stop a tracee. If the tracee is running or sleeping in kernel
space and PTRACE_SYSCALL is in effect, the system call is
interrupted and syscall-exit-stop is reported. (The
interrupted system call is restarted when the tracee is
restarted.) If the tracee was already stopped by a signal and
PTRACE_LISTEN was sent to it, the tracee stops with
PTRACE_EVENT_STOP and WSTOPSIG(status) returns the stop
signal. If any other ptrace-stop is generated at the same
time (for example, if a signal is sent to the tracee), this
ptrace-stop happens. If none of the above applies (for
example, if the tracee is running in user space), it stops
with PTRACE_EVENT_STOP with WSTOPSIG(status) == SIGTRAP.
PTRACE_INTERRUPT only works on tracees attached by
PTRACE_SEIZE.
PTRACE_ATTACH
Attach to the process specified in pid, making it a tracee of
the calling process. The tracee is sent a SIGSTOP, but will
not necessarily have stopped by the completion of this call;
use waitpid(2) to wait for the tracee to stop. See the
"Attaching and detaching" subsection for additional
information. (addr and data are ignored.)
Permission to perform a PTRACE_ATTACH is governed by a ptrace
access mode PTRACE_MODE_ATTACH_REALCREDS check; see below.
PTRACE_SEIZE (since Linux 3.4)
Attach to the process specified in pid, making it a tracee of
the calling process. Unlike PTRACE_ATTACH, PTRACE_SEIZE does
not stop the process. Group-stops are reported as
PTRACE_EVENT_STOP and WSTOPSIG(status) returns the stop
signal. Automatically attached children stop with
PTRACE_EVENT_STOP and WSTOPSIG(status) returns SIGTRAP instead
of having SIGSTOP signal delivered to them. execve(2) does
not deliver an extra SIGTRAP. Only a PTRACE_SEIZEd process
can accept PTRACE_INTERRUPT and PTRACE_LISTEN commands. The
"seized" behavior just described is inherited by children that
are automatically attached using PTRACE_O_TRACEFORK,
PTRACE_O_TRACEVFORK, and PTRACE_O_TRACECLONE. addr must be
zero. data contains a bit mask of ptrace options to activate
immediately.
Permission to perform a PTRACE_SEIZE is governed by a ptrace
access mode PTRACE_MODE_ATTACH_REALCREDS check; see below.
PTRACE_DETACH
Restart the stopped tracee as for PTRACE_CONT, but first
detach from it. Under Linux, a tracee can be detached in this
way regardless of which method was used to initiate tracing.
(addr is ignored.)
Death under ptrace
When a (possibly multithreaded) process receives a killing signal
(one whose disposition is set to SIG_DFL and whose default action is
to kill the process), all threads exit. Tracees report their death
to their tracer(s). Notification of this event is delivered via
waitpid(2).
Note that the killing signal will first cause signal-delivery-stop
(on one tracee only), and only after it is injected by the tracer (or
after it was dispatched to a thread which isn't traced), will death
from the signal happen on all tracees within a multithreaded process.
(The term "signal-delivery-stop" is explained below.)
SIGKILL does not generate signal-delivery-stop and therefore the
tracer can't suppress it. SIGKILL kills even within system calls
(syscall-exit-stop is not generated prior to death by SIGKILL). The
net effect is that SIGKILL always kills the process (all its
threads), even if some threads of the process are ptraced.
When the tracee calls _exit(2), it reports its death to its tracer.
Other threads are not affected.
When any thread executes exit_group(2), every tracee in its thread
group reports its death to its tracer.
If the PTRACE_O_TRACEEXIT option is on, PTRACE_EVENT_EXIT will happen
before actual death. This applies to exits via exit(2),
exit_group(2), and signal deaths (except SIGKILL, depending on the
kernel version; see BUGS below), and when threads are torn down on
execve(2) in a multithreaded process.
The tracer cannot assume that the ptrace-stopped tracee exists.
There are many scenarios when the tracee may die while stopped (such
as SIGKILL). Therefore, the tracer must be prepared to handle an
ESRCH error on any ptrace operation. Unfortunately, the same error
is returned if the tracee exists but is not ptrace-stopped (for
commands which require a stopped tracee), or if it is not traced by
the process which issued the ptrace call. The tracer needs to keep
track of the stopped/running state of the tracee, and interpret ESRCH
as "tracee died unexpectedly" only if it knows that the tracee has
been observed to enter ptrace-stop. Note that there is no guarantee
that waitpid(WNOHANG) will reliably report the tracee's death status
if a ptrace operation returned ESRCH. waitpid(WNOHANG) may return 0
instead. In other words, the tracee may be "not yet fully dead", but
already refusing ptrace requests.
The tracer can't assume that the tracee always ends its life by
reporting WIFEXITED(status) or WIFSIGNALED(status); there are cases
where this does not occur. For example, if a thread other than
thread group leader does an execve(2), it disappears; its PID will
never be seen again, and any subsequent ptrace stops will be reported
under the thread group leader's PID.
Stopped states
A tracee can be in two states: running or stopped. For the purposes
of ptrace, a tracee which is blocked in a system call (such as
read(2), pause(2), etc.) is nevertheless considered to be running,
even if the tracee is blocked for a long time. The state of the
tracee after PTRACE_LISTEN is somewhat of a gray area: it is not in
any ptrace-stop (ptrace commands won't work on it, and it will
deliver waitpid(2) notifications), but it also may be considered
"stopped" because it is not executing instructions (is not
scheduled), and if it was in group-stop before PTRACE_LISTEN, it will
not respond to signals until SIGCONT is received.
There are many kinds of states when the tracee is stopped, and in
ptrace discussions they are often conflated. Therefore, it is
important to use precise terms.
In this manual page, any stopped state in which the tracee is ready
to accept ptrace commands from the tracer is called ptrace-stop.
Ptrace-stops can be further subdivided into signal-delivery-stop,
group-stop, syscall-stop, and so on. These stopped states are
described in detail below.
When the running tracee enters ptrace-stop, it notifies its tracer
using waitpid(2) (or one of the other "wait" system calls). Most of
this manual page assumes that the tracer waits with:
pid = waitpid(pid_or_minus_1, &status, __WALL);
Ptrace-stopped tracees are reported as returns with pid greater than
0 and WIFSTOPPED(status) true.
The __WALL flag does not include the WSTOPPED and WEXITED flags, but
implies their functionality.
Setting the WCONTINUED flag when calling waitpid(2) is not
recommended: the "continued" state is per-process and consuming it
can confuse the real parent of the tracee.
Use of the WNOHANG flag may cause waitpid(2) to return 0 ("no wait
results available yet") even if the tracer knows there should be a
notification. Example:
errno = 0;
ptrace(PTRACE_CONT, pid, 0L, 0L);
if (errno == ESRCH) {
/* tracee is dead */
r = waitpid(tracee, &status, __WALL | WNOHANG);
/* r can still be 0 here! */
}
The following kinds of ptrace-stops exist: signal-delivery-stops,
group-stops, PTRACE_EVENT stops, syscall-stops. They all are
reported by waitpid(2) with WIFSTOPPED(status) true. They may be
differentiated by examining the value status>>8, and if there is
ambiguity in that value, by querying PTRACE_GETSIGINFO. (Note: the
WSTOPSIG(status) macro can't be used to perform this examination,
because it returns the value (status>>8) & 0xff.)
Signal-delivery-stop
When a (possibly multithreaded) process receives any signal except
SIGKILL, the kernel selects an arbitrary thread which handles the
signal. (If the signal is generated with tgkill(2), the target
thread can be explicitly selected by the caller.) If the selected
thread is traced, it enters signal-delivery-stop. At this point, the
signal is not yet delivered to the process, and can be suppressed by
the tracer. If the tracer doesn't suppress the signal, it passes the
signal to the tracee in the next ptrace restart request. This second
step of signal delivery is called signal injection in this manual
page. Note that if the signal is blocked, signal-delivery-stop
doesn't happen until the signal is unblocked, with the usual
exception that SIGSTOP can't be blocked.
Signal-delivery-stop is observed by the tracer as waitpid(2)
returning with WIFSTOPPED(status) true, with the signal returned by
WSTOPSIG(status). If the signal is SIGTRAP, this may be a different
kind of ptrace-stop; see the "Syscall-stops" and "execve" sections
below for details. If WSTOPSIG(status) returns a stopping signal,
this may be a group-stop; see below.
Signal injection and suppression
After signal-delivery-stop is observed by the tracer, the tracer
should restart the tracee with the call
ptrace(PTRACE_restart, pid, 0, sig)
where PTRACE_restart is one of the restarting ptrace requests. If
sig is 0, then a signal is not delivered. Otherwise, the signal sig
is delivered. This operation is called signal injection in this
manual page, to distinguish it from signal-delivery-stop.
The sig value may be different from the WSTOPSIG(status) value: the
tracer can cause a different signal to be injected.
Note that a suppressed signal still causes system calls to return
prematurely. In this case, system calls will be restarted: the
tracer will observe the tracee to reexecute the interrupted system
call (or restart_syscall(2) system call for a few system calls which
use a different mechanism for restarting) if the tracer uses
PTRACE_SYSCALL. Even system calls (such as poll(2)) which are not
restartable after signal are restarted after signal is suppressed;
however, kernel bugs exist which cause some system calls to fail with
EINTR even though no observable signal is injected to the tracee.
Restarting ptrace commands issued in ptrace-stops other than signal-
delivery-stop are not guaranteed to inject a signal, even if sig is
nonzero. No error is reported; a nonzero sig may simply be ignored.
Ptrace users should not try to "create a new signal" this way: use
tgkill(2) instead.
The fact that signal injection requests may be ignored when
restarting the tracee after ptrace stops that are not signal-
delivery-stops is a cause of confusion among ptrace users. One
typical scenario is that the tracer observes group-stop, mistakes it
for signal-delivery-stop, restarts the tracee with
ptrace(PTRACE_restart, pid, 0, stopsig)
with the intention of injecting stopsig, but stopsig gets ignored and
the tracee continues to run.
The SIGCONT signal has a side effect of waking up (all threads of) a
group-stopped process. This side effect happens before signal-
delivery-stop. The tracer can't suppress this side effect (it can
only suppress signal injection, which only causes the SIGCONT handler
to not be executed in the tracee, if such a handler is installed).
In fact, waking up from group-stop may be followed by signal-
delivery-stop for signal(s) other thanSIGCONT, if they were pending
when SIGCONT was delivered. In other words, SIGCONT may be not the
first signal observed by the tracee after it was sent.
Stopping signals cause (all threads of) a process to enter group-
stop. This side effect happens after signal injection, and therefore
can be suppressed by the tracer.
In Linux 2.4 and earlier, the SIGSTOP signal can't be injected.
PTRACE_GETSIGINFO can be used to retrieve a siginfo_t structure which
corresponds to the delivered signal. PTRACE_SETSIGINFO may be used
to modify it. If PTRACE_SETSIGINFO has been used to alter siginfo_t,
the si_signo field and the sig parameter in the restarting command
must match, otherwise the result is undefined.
Group-stop
When a (possibly multithreaded) process receives a stopping signal,
all threads stop. If some threads are traced, they enter a group-
stop. Note that the stopping signal will first cause signal-
delivery-stop (on one tracee only), and only after it is injected by
the tracer (or after it was dispatched to a thread which isn't
traced), will group-stop be initiated on all tracees within the
multithreaded process. As usual, every tracee reports its group-stop
separately to the corresponding tracer.
Group-stop is observed by the tracer as waitpid(2) returning with
WIFSTOPPED(status) true, with the stopping signal available via
WSTOPSIG(status). The same result is returned by some other classes
of ptrace-stops, therefore the recommended practice is to perform the
call
ptrace(PTRACE_GETSIGINFO, pid, 0, &siginfo)
The call can be avoided if the signal is not SIGSTOP, SIGTSTP,
SIGTTIN, or SIGTTOU; only these four signals are stopping signals.
If the tracer sees something else, it can't be a group-stop.
Otherwise, the tracer needs to call PTRACE_GETSIGINFO. If
PTRACE_GETSIGINFO fails with EINVAL, then it is definitely a group-
stop. (Other failure codes are possible, such as ESRCH ("no such
process") if a SIGKILL killed the tracee.)
If tracee was attached using PTRACE_SEIZE, group-stop is indicated by
PTRACE_EVENT_STOP: status>>16 == PTRACE_EVENT_STOP. This allows
detection of group-stops without requiring an extra PTRACE_GETSIGINFO
call.
As of Linux 2.6.38, after the tracer sees the tracee ptrace-stop and
until it restarts or kills it, the tracee will not run, and will not
send notifications (except SIGKILL death) to the tracer, even if the
tracer enters into another waitpid(2) call.
The kernel behavior described in the previous paragraph causes a
problem with transparent handling of stopping signals. If the tracer
restarts the tracee after group-stop, the stopping signal is
effectively ignored—the tracee doesn't remain stopped, it runs. If
the tracer doesn't restart the tracee before entering into the next
waitpid(2), future SIGCONT signals will not be reported to the
tracer; this would cause the SIGCONT signals to have no effect on the
tracee.
Since Linux 3.4, there is a method to overcome this problem: instead
of PTRACE_CONT, a PTRACE_LISTEN command can be used to restart a
tracee in a way where it does not execute, but waits for a new event
which it can report via waitpid(2) (such as when it is restarted by a
SIGCONT).
PTRACE_EVENT stops
If the tracer sets PTRACE_O_TRACE_* options, the tracee will enter
ptrace-stops called PTRACE_EVENT stops.
PTRACE_EVENT stops are observed by the tracer as waitpid(2) returning
with WIFSTOPPED(status), and WSTOPSIG(status) returns SIGTRAP. An
additional bit is set in the higher byte of the status word: the
value status>>8 will be
(SIGTRAP | PTRACE_EVENT_foo << 8).
The following events exist:
PTRACE_EVENT_VFORK
Stop before return from vfork(2) or clone(2) with the
CLONE_VFORK flag. When the tracee is continued after this
stop, it will wait for child to exit/exec before continuing
its execution (in other words, the usual behavior on
vfork(2)).
PTRACE_EVENT_FORK
Stop before return from fork(2) or clone(2) with the exit
signal set to SIGCHLD.
PTRACE_EVENT_CLONE
Stop before return from clone(2).
PTRACE_EVENT_VFORK_DONE
Stop before return from vfork(2) or clone(2) with the
CLONE_VFORK flag, but after the child unblocked this tracee by
exiting or execing.
For all four stops described above, the stop occurs in the parent
(i.e., the tracee), not in the newly created thread.
PTRACE_GETEVENTMSG can be used to retrieve the new thread's ID.
PTRACE_EVENT_EXEC
Stop before return from execve(2). Since Linux 3.0,
PTRACE_GETEVENTMSG returns the former thread ID.
PTRACE_EVENT_EXIT
Stop before exit (including death from exit_group(2)), signal
death, or exit caused by execve(2) in a multithreaded process.
PTRACE_GETEVENTMSG returns the exit status. Registers can be
examined (unlike when "real" exit happens). The tracee is
still alive; it needs to be PTRACE_CONTed or PTRACE_DETACHed
to finish exiting.
PTRACE_EVENT_STOP
Stop induced by PTRACE_INTERRUPT command, or group-stop, or
initial ptrace-stop when a new child is attached (only if
attached using PTRACE_SEIZE).
PTRACE_EVENT_SECCOMP
Stop triggered by a seccomp(2) rule on tracee syscall entry
when PTRACE_O_TRACESECCOMP has been set by the tracer. The
seccomp event message data (from the SECCOMP_RET_DATA portion
of the seccomp filter rule) can be retrieved with
PTRACE_GETEVENTMSG.
PTRACE_GETSIGINFO on PTRACE_EVENT stops returns SIGTRAP in si_signo,
with si_code set to (event<<8) | SIGTRAP.
Syscall-stops
If the tracee was restarted by PTRACE_SYSCALL, the tracee enters
syscall-enter-stop just prior to entering any system call. If the
tracer restarts the tracee with PTRACE_SYSCALL, the tracee enters
syscall-exit-stop when the system call is finished, or if it is
interrupted by a signal. (That is, signal-delivery-stop never
happens between syscall-enter-stop and syscall-exit-stop; it happens
after syscall-exit-stop.)
Other possibilities are that the tracee may stop in a PTRACE_EVENT
stop, exit (if it entered _exit(2) or exit_group(2)), be killed by
SIGKILL, or die silently (if it is a thread group leader, the
execve(2) happened in another thread, and that thread is not traced
by the same tracer; this situation is discussed later).
Syscall-enter-stop and syscall-exit-stop are observed by the tracer
as waitpid(2) returning with WIFSTOPPED(status) true, and
WSTOPSIG(status) giving SIGTRAP. If the PTRACE_O_TRACESYSGOOD option
was set by the tracer, then WSTOPSIG(status) will give the value
(SIGTRAP | 0x80).
Syscall-stops can be distinguished from signal-delivery-stop with
SIGTRAP by querying PTRACE_GETSIGINFO for the following cases:
si_code <= 0
SIGTRAP was delivered as a result of a user-space action, for
example, a system call (tgkill(2), kill(2), sigqueue(3),
etc.), expiration of a POSIX timer, change of state on a POSIX
message queue, or completion of an asynchronous I/O request.
si_code == SI_KERNEL (0x80)
SIGTRAP was sent by the kernel.
si_code == SIGTRAP or si_code == (SIGTRAP|0x80)
This is a syscall-stop.
However, syscall-stops happen very often (twice per system call), and
performing PTRACE_GETSIGINFO for every syscall-stop may be somewhat
expensive.
Some architectures allow the cases to be distinguished by examining
registers. For example, on x86, rax == -ENOSYS in syscall-enter-
stop. Since SIGTRAP (like any other signal) always happens after
syscall-exit-stop, and at this point rax almost never contains
-ENOSYS, the SIGTRAP looks like "syscall-stop which is not syscall-
enter-stop"; in other words, it looks like a "stray syscall-exit-
stop" and can be detected this way. But such detection is fragile
and is best avoided.
Using the PTRACE_O_TRACESYSGOOD option is the recommended method to
distinguish syscall-stops from other kinds of ptrace-stops, since it
is reliable and does not incur a performance penalty.
Syscall-enter-stop and syscall-exit-stop are indistinguishable from
each other by the tracer. The tracer needs to keep track of the
sequence of ptrace-stops in order to not misinterpret syscall-enter-
stop as syscall-exit-stop or vice versa. The rule is that syscall-
enter-stop is always followed by syscall-exit-stop, PTRACE_EVENT stop
or the tracee's death; no other kinds of ptrace-stop can occur in
between.
If after syscall-enter-stop, the tracer uses a restarting command
other than PTRACE_SYSCALL, syscall-exit-stop is not generated.
PTRACE_GETSIGINFO on syscall-stops returns SIGTRAP in si_signo, with
si_code set to SIGTRAP or (SIGTRAP|0x80).
PTRACE_SINGLESTEP, PTRACE_SYSEMU, PTRACE_SYSEMU_SINGLESTEP stops
[Details of these kinds of stops are yet to be documented.]
Informational and restarting ptrace commands
Most ptrace commands (all except PTRACE_ATTACH, PTRACE_SEIZE,
PTRACE_TRACEME, PTRACE_INTERRUPT, and PTRACE_KILL) require the tracee
to be in a ptrace-stop, otherwise they fail with ESRCH.
When the tracee is in ptrace-stop, the tracer can read and write data
to the tracee using informational commands. These commands leave the
tracee in ptrace-stopped state:
ptrace(PTRACE_PEEKTEXT/PEEKDATA/PEEKUSER, pid, addr, 0);
ptrace(PTRACE_POKETEXT/POKEDATA/POKEUSER, pid, addr, long_val);
ptrace(PTRACE_GETREGS/GETFPREGS, pid, 0, &struct);
ptrace(PTRACE_SETREGS/SETFPREGS, pid, 0, &struct);
ptrace(PTRACE_GETREGSET, pid, NT_foo, &iov);
ptrace(PTRACE_SETREGSET, pid, NT_foo, &iov);
ptrace(PTRACE_GETSIGINFO, pid, 0, &siginfo);
ptrace(PTRACE_SETSIGINFO, pid, 0, &siginfo);
ptrace(PTRACE_GETEVENTMSG, pid, 0, &long_var);
ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_flags);
Note that some errors are not reported. For example, setting signal
information (siginfo) may have no effect in some ptrace-stops, yet
the call may succeed (return 0 and not set errno); querying
PTRACE_GETEVENTMSG may succeed and return some random value if
current ptrace-stop is not documented as returning a meaningful event
message.
The call
ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_flags);
affects one tracee. The tracee's current flags are replaced. Flags
are inherited by new tracees created and "auto-attached" via active
PTRACE_O_TRACEFORK, PTRACE_O_TRACEVFORK, or PTRACE_O_TRACECLONE
options.
Another group of commands makes the ptrace-stopped tracee run. They
have the form:
ptrace(cmd, pid, 0, sig);
where cmd is PTRACE_CONT, PTRACE_LISTEN, PTRACE_DETACH,
PTRACE_SYSCALL, PTRACE_SINGLESTEP, PTRACE_SYSEMU, or
PTRACE_SYSEMU_SINGLESTEP. If the tracee is in signal-delivery-stop,
sig is the signal to be injected (if it is nonzero). Otherwise, sig
may be ignored. (When restarting a tracee from a ptrace-stop other
than signal-delivery-stop, recommended practice is to always pass 0
in sig.)
Attaching and detaching
A thread can be attached to the tracer using the call
ptrace(PTRACE_ATTACH, pid, 0, 0);
or
ptrace(PTRACE_SEIZE, pid, 0, PTRACE_O_flags);
PTRACE_ATTACH sends SIGSTOP to this thread. If the tracer wants this
SIGSTOP to have no effect, it needs to suppress it. Note that if
other signals are concurrently sent to this thread during attach, the
tracer may see the tracee enter signal-delivery-stop with other
signal(s) first! The usual practice is to reinject these signals
until SIGSTOP is seen, then suppress SIGSTOP injection. The design
bug here is that a ptrace attach and a concurrently delivered SIGSTOP
may race and the concurrent SIGSTOP may be lost.
Since attaching sends SIGSTOP and the tracer usually suppresses it,
this may cause a stray EINTR return from the currently executing
system call in the tracee, as described in the "Signal injection and
suppression" section.
Since Linux 3.4, PTRACE_SEIZE can be used instead of PTRACE_ATTACH.
PTRACE_SEIZE does not stop the attached process. If you need to stop
it after attach (or at any other time) without sending it any
signals, use PTRACE_INTERRUPT command.
The request
ptrace(PTRACE_TRACEME, 0, 0, 0);
turns the calling thread into a tracee. The thread continues to run
(doesn't enter ptrace-stop). A common practice is to follow the
PTRACE_TRACEME with
raise(SIGSTOP);
and allow the parent (which is our tracer now) to observe our signal-
delivery-stop.
If the PTRACE_O_TRACEFORK, PTRACE_O_TRACEVFORK, or
PTRACE_O_TRACECLONE options are in effect, then children created by,
respectively, vfork(2) or clone(2) with the CLONE_VFORK flag, fork(2)
or clone(2) with the exit signal set to SIGCHLD, and other kinds of
clone(2), are automatically attached to the same tracer which traced
their parent. SIGSTOP is delivered to the children, causing them to
enter signal-delivery-stop after they exit the system call which
created them.
Detaching of the tracee is performed by:
ptrace(PTRACE_DETACH, pid, 0, sig);
PTRACE_DETACH is a restarting operation; therefore it requires the
tracee to be in ptrace-stop. If the tracee is in signal-delivery-
stop, a signal can be injected. Otherwise, the sig parameter may be
silently ignored.
If the tracee is running when the tracer wants to detach it, the
usual solution is to send SIGSTOP (using tgkill(2), to make sure it
goes to the correct thread), wait for the tracee to stop in signal-
delivery-stop for SIGSTOP and then detach it (suppressing SIGSTOP
injection). A design bug is that this can race with concurrent
SIGSTOPs. Another complication is that the tracee may enter other
ptrace-stops and needs to be restarted and waited for again, until
SIGSTOP is seen. Yet another complication is to be sure that the
tracee is not already ptrace-stopped, because no signal delivery
happens while it is—not even SIGSTOP.
If the tracer dies, all tracees are automatically detached and
restarted, unless they were in group-stop. Handling of restart from
group-stop is currently buggy, but the "as planned" behavior is to
leave tracee stopped and waiting for SIGCONT. If the tracee is
restarted from signal-delivery-stop, the pending signal is injected.
execve(2) under ptrace
When one thread in a multithreaded process calls execve(2), the
kernel destroys all other threads in the process, and resets the
thread ID of the execing thread to the thread group ID (process ID).
(Or, to put things another way, when a multithreaded process does an
execve(2), at completion of the call, it appears as though the
execve(2) occurred in the thread group leader, regardless of which
thread did the execve(2).) This resetting of the thread ID looks
very confusing to tracers:
* All other threads stop in PTRACE_EVENT_EXIT stop, if the
PTRACE_O_TRACEEXIT option was turned on. Then all other threads
except the thread group leader report death as if they exited via
_exit(2) with exit code 0.
* The execing tracee changes its thread ID while it is in the
execve(2). (Remember, under ptrace, the "pid" returned from
waitpid(2), or fed into ptrace calls, is the tracee's thread ID.)
That is, the tracee's thread ID is reset to be the same as its
process ID, which is the same as the thread group leader's thread
ID.
* Then a PTRACE_EVENT_EXEC stop happens, if the PTRACE_O_TRACEEXEC
option was turned on.
* If the thread group leader has reported its PTRACE_EVENT_EXIT stop
by this time, it appears to the tracer that the dead thread leader
"reappears from nowhere". (Note: the thread group leader does not
report death via WIFEXITED(status) until there is at least one
other live thread. This eliminates the possibility that the
tracer will see it dying and then reappearing.) If the thread
group leader was still alive, for the tracer this may look as if
thread group leader returns from a different system call than it
entered, or even "returned from a system call even though it was
not in any system call". If the thread group leader was not
traced (or was traced by a different tracer), then during
execve(2) it will appear as if it has become a tracee of the
tracer of the execing tracee.
All of the above effects are the artifacts of the thread ID change in
the tracee.
The PTRACE_O_TRACEEXEC option is the recommended tool for dealing
with this situation. First, it enables PTRACE_EVENT_EXEC stop, which
occurs before execve(2) returns. In this stop, the tracer can use
PTRACE_GETEVENTMSG to retrieve the tracee's former thread ID. (This
feature was introduced in Linux 3.0.) Second, the PTRACE_O_TRACEEXEC
option disables legacy SIGTRAP generation on execve(2).
When the tracer receives PTRACE_EVENT_EXEC stop notification, it is
guaranteed that except this tracee and the thread group leader, no
other threads from the process are alive.
On receiving the PTRACE_EVENT_EXEC stop notification, the tracer
should clean up all its internal data structures describing the
threads of this process, and retain only one data structure—one which
describes the single still running tracee, with
thread ID == thread group ID == process ID.
Example: two threads call execve(2) at the same time:
*** we get syscall-enter-stop in thread 1: **
PID1 execve("/bin/foo", "foo" <unfinished ...>
*** we issue PTRACE_SYSCALL for thread 1 **
*** we get syscall-enter-stop in thread 2: **
PID2 execve("/bin/bar", "bar" <unfinished ...>
*** we issue PTRACE_SYSCALL for thread 2 **
*** we get PTRACE_EVENT_EXEC for PID0, we issue PTRACE_SYSCALL **
*** we get syscall-exit-stop for PID0: **
PID0 <... execve resumed> ) = 0
If the PTRACE_O_TRACEEXEC option is not in effect for the execing
tracee, and if the tracee was PTRACE_ATTACHed rather that
PTRACE_SEIZEd, the kernel delivers an extra SIGTRAP to the tracee
after execve(2) returns. This is an ordinary signal (similar to one
which can be generated by kill -TRAP), not a special kind of ptrace-
stop. Employing PTRACE_GETSIGINFO for this signal returns si_code
set to 0 (SI_USER). This signal may be blocked by signal mask, and
thus may be delivered (much) later.
Usually, the tracer (for example, strace(1)) would not want to show
this extra post-execve SIGTRAP signal to the user, and would suppress
its delivery to the tracee (if SIGTRAP is set to SIG_DFL, it is a
killing signal). However, determining whichSIGTRAP to suppress is
not easy. Setting the PTRACE_O_TRACEEXEC option or using
PTRACE_SEIZE and thus suppressing this extra SIGTRAP is the
recommended approach.
Real parent
The ptrace API (ab)uses the standard UNIX parent/child signaling over
waitpid(2). This used to cause the real parent of the process to
stop receiving several kinds of waitpid(2) notifications when the
child process is traced by some other process.
Many of these bugs have been fixed, but as of Linux 2.6.38 several
still exist; see BUGS below.
As of Linux 2.6.38, the following is believed to work correctly:
* exit/death by signal is reported first to the tracer, then, when
the tracer consumes the waitpid(2) result, to the real parent (to
the real parent only when the whole multithreaded process exits).
If the tracer and the real parent are the same process, the report
is sent only once.

On success, the PTRACE_PEEK* requests return the requested data (but
see NOTES), while other requests return zero.
On error, all requests return -1, and errno is set appropriately.
Since the value returned by a successful PTRACE_PEEK* request may be
-1, the caller must clear errno before the call, and then check it
afterward to determine whether or not an error occurred.

EBUSY (i386 only) There was an error with allocating or freeing a
debug register.
EFAULT There was an attempt to read from or write to an invalid area
in the tracer's or the tracee's memory, probably because the
area wasn't mapped or accessible. Unfortunately, under Linux,
different variations of this fault will return EIO or EFAULT
more or less arbitrarily.
EINVAL An attempt was made to set an invalid option.
EIO request is invalid, or an attempt was made to read from or
write to an invalid area in the tracer's or the tracee's
memory, or there was a word-alignment violation, or an invalid
signal was specified during a restart request.
EPERM The specified process cannot be traced. This could be because
the tracer has insufficient privileges (the required
capability is CAP_SYS_PTRACE); unprivileged processes cannot
trace processes that they cannot send signals to or those
running set-user-ID/set-group-ID programs, for obvious
reasons. Alternatively, the process may already be being
traced, or (on kernels before 2.6.26) be init(1) (PID 1).
ESRCH The specified process does not exist, or is not currently
being traced by the caller, or is not stopped (for requests
that require a stopped tracee).

Although arguments to ptrace() are interpreted according to the
prototype given, glibc currently declares ptrace() as a variadic
function with only the request argument fixed. It is recommended to
always supply four arguments, even if the requested operation does
not use them, setting unused/ignored arguments to 0L or (void *) 0.
In Linux kernels before 2.6.26, init(1), the process with PID 1, may
not be traced.
A tracees parent continues to be the tracer even if that tracer calls
execve(2).
The layout of the contents of memory and the USER area are quite
operating-system- and architecture-specific. The offset supplied,
and the data returned, might not entirely match with the definition
of struct user.
The size of a "word" is determined by the operating-system variant
(e.g., for 32-bit Linux it is 32 bits).
This page documents the way the ptrace() call works currently in
Linux. Its behavior differs significantly on other flavors of UNIX.
In any case, use of ptrace() is highly specific to the operating
system and architecture.
Ptrace access mode checking
Various parts of the kernel-user-space API (not just ptrace()
operations), require so-called "ptrace access mode" checks, whose
outcome determines whether an operation is permitted (or, in a few
cases, causes a "read" operation to return sanitized data). These
checks are performed in cases where one process can inspect sensitive
information about, or in some cases modify the state of, another
process. The checks are based on factors such as the credentials and
capabilities of the two processes, whether or not the "target"
process is dumpable, and the results of checks performed by any
enabled Linux Security Module (LSM)—for example, SELinux, Yama, or
Smack—and by the commoncap LSM (which is always invoked).
Prior to Linux 2.6.27, all access checks were of a single type.
Since Linux 2.6.27, two access mode levels are distinguished:
PTRACE_MODE_READ
For "read" operations or other operations that are less
dangerous, such as: get_robust_list(2); kcmp(2); reading
/proc/[pid]/auxv, /proc/[pid]/environ, or /proc/[pid]/stat; or
readlink(2) of a /proc/[pid]/ns/* file.
PTRACE_MODE_ATTACH
For "write" operations, or other operations that are more
dangerous, such as: ptrace attaching (PTRACE_ATTACH) to
another process or calling process_vm_writev(2).
(PTRACE_MODE_ATTACH was effectively the default before Linux
2.6.27.)
Since Linux 4.5, the above access mode checks are combined (ORed)
with one of the following modifiers:
PTRACE_MODE_FSCREDS
Use the caller's filesystem UID and GID (see credentials(7))
or effective capabilities for LSM checks.
PTRACE_MODE_REALCREDS
Use the caller's real UID and GID or permitted capabilities
for LSM checks. This was effectively the default before Linux
4.5.
Because combining one of the credential modifiers with one of the
aforementioned access modes is typical, some macros are defined in
the kernel sources for the combinations:
PTRACE_MODE_READ_FSCREDS
Defined as PTRACE_MODE_READ | PTRACE_MODE_FSCREDS.
PTRACE_MODE_READ_REALCREDS
Defined as PTRACE_MODE_READ | PTRACE_MODE_REALCREDS.
PTRACE_MODE_ATTACH_FSCREDS
Defined as PTRACE_MODE_ATTACH | PTRACE_MODE_FSCREDS.
PTRACE_MODE_ATTACH_REALCREDS
Defined as PTRACE_MODE_ATTACH | PTRACE_MODE_REALCREDS.
One further modifier can be ORed with the access mode:
PTRACE_MODE_NOAUDIT (since Linux 3.3)
Don't audit this access mode check. This modifier is employed
for ptrace access mode checks (such as checks when reading
/proc/[pid]/stat) that merely cause the output to be filtered
or sanitized, rather than causing an error to be returned to
the caller. In these cases, accessing the file is not a
security violation and there is no reason to generate a
security audit record. This modifier suppresses the
generation of such an audit record for the particular access
check.
Note that all of the PTRACE_MODE_* constants described in this
subsection are kernel-internal, and not visible to user space. The
constant names are mentioned here in order to label the various kinds
of ptrace access mode checks that are performed for various system
calls and accesses to various pseudofiles (e.g., under /proc). These
names are used in other manual pages to provide a simple shorthand
for labeling the different kernel checks.
The algorithm employed for ptrace access mode checking determines
whether the calling process is allowed to perform the corresponding
action on the target process. (In the case of opening /proc/[pid]
files, the "calling process" is the one opening the file, and the
process with the corresponding PID is the "target process".) The
algorithm is as follows:
1. If the calling thread and the target thread are in the same
thread group, access is always allowed.
2. If the access mode specifies PTRACE_MODE_FSCREDS, then, for the
check in the next step, employ the caller's filesystem UID and
GID. (As noted in credentials(7), the filesystem UID and GID
almost always have the same values as the corresponding effective
IDs.)
Otherwise, the access mode specifies PTRACE_MODE_REALCREDS, so
use the caller's real UID and GID for the checks in the next
step. (Most APIs that check the caller's UID and GID use the
effective IDs. For historical reasons, the PTRACE_MODE_REALCREDS
check uses the real IDs instead.)
3. Deny access if neither of the following is true:
· The real, effective, and saved-set user IDs of the target match
the caller's user ID, and the real, effective, and saved-set
group IDs of the target match the caller's group ID.
· The caller has the CAP_SYS_PTRACE capability in the user
namespace of the target.
4. Deny access if the target process "dumpable" attribute has a
value other than 1 (SUID_DUMP_USER; see the discussion of
PR_SET_DUMPABLE in prctl(2)), and the caller does not have the
CAP_SYS_PTRACE capability in the user namespace of the target
process.
5. The kernel LSM security_ptrace_access_check() interface is
invoked to see if ptrace access is permitted. The results depend
on the LSM(s). The implementation of this interface in the
commoncap LSM performs the following steps:
a) If the access mode includes PTRACE_MODE_FSCREDS, then use the
caller's effective capability set in the following check;
otherwise (the access mode specifies PTRACE_MODE_REALCREDS,
so) use the caller's permitted capability set.
b) Deny access if neither of the following is true:
· The caller and the target process are in the same user
namespace, and the caller's capabilities are a proper
superset of the target process's permitted capabilities.
· The caller has the CAP_SYS_PTRACE capability in the target
process's user namespace.
Note that the commoncap LSM does not distinguish between
PTRACE_MODE_READ and PTRACE_MODE_ATTACH.
6. If access has not been denied by any of the preceding steps, then
access is allowed.
/proc/sys/kernel/yama/ptrace_scope
On systems with the Yama Linux Security Module (LSM) installed (i.e.,
the kernel was configured with CONFIG_SECURITY_YAMA), the
/proc/sys/kernel/yama/ptrace_scope file (available since Linux 3.4)
can be used to restrict the ability to trace a process with ptrace()
(and thus also the ability to use tools such as strace(1) and
gdb(1)). The goal of such restrictions is to prevent attack
escalation whereby a compromised process can ptrace-attach to other
sensitive processes (e.g., a GPG agent or an SSH session) owned by
the user in order to gain additional credentials that may exist in
memory and thus expand the scope of the attack.
More precisely, the Yama LSM limits two types of operations:
* Any operation that performs a ptrace access mode
PTRACE_MODE_ATTACH check—for example, ptrace() PTRACE_ATTACH.
(See the "Ptrace access mode checking" discussion above.)
* ptrace() PTRACE_TRACEME.
A process that has the CAP_SYS_PTRACE capability can update the
/proc/sys/kernel/yama/ptrace_scope file with one of the following
values:
0 ("classic ptrace permissions")
No additional restrictions on operations that perform
PTRACE_MODE_ATTACH checks (beyond those imposed by the
commoncap and other LSMs).
The use of PTRACE_TRACEME is unchanged.
1 ("restricted ptrace") [default value]
When performing an operation that requires a
PTRACE_MODE_ATTACH check, the calling process must either have
the CAP_SYS_PTRACE capability in the user namespace of the
target process or it must have a predefined relationship with
the target process. By default, the predefined relationship
is that the target process must be a descendant of the caller.
A target process can employ the prctl(2)PR_SET_PTRACER
operation to declare an additional PID that is allowed to
perform PTRACE_MODE_ATTACH operations on the target. See the
kernel source file Documentation/security/Yama.txt for further
details.
The use of PTRACE_TRACEME is unchanged.
2 ("admin-only attach")
Only processes with the CAP_SYS_PTRACE capability in the user
namespace of the target process may perform PTRACE_MODE_ATTACH
operations or trace children that employ PTRACE_TRACEME.
3 ("no attach")
No process may perform PTRACE_MODE_ATTACH operations or trace
children that employ PTRACE_TRACEME.
Once this value has been written to the file, it cannot be
changed.
With respect to values 1 and 2, note that creating a new user
namespace effectively removes the protection offered by Yama. This
is because a process in the parent user namespace whose effective UID
matches the UID of the creator of a child namespace has all
capabilities (including CAP_SYS_PTRACE) when performing operations
within the child user namespace (and further-removed descendants of
that namespace). Consequently, when a process tries to use user
namespaces to sandbox itself, it inadvertently weakens the
protections offered by the Yama LSM.
C library/kernel differences
At the system call level, the PTRACE_PEEKTEXT, PTRACE_PEEKDATA, and
PTRACE_PEEKUSER requests have a different API: they store the result
at the address specified by the data parameter, and the return value
is the error flag. The glibc wrapper function provides the API given
in DESCRIPTION above, with the result being returned via the function
return value.

On hosts with 2.6 kernel headers, PTRACE_SETOPTIONS is declared with
a different value than the one for 2.4. This leads to applications
compiled with 2.6 kernel headers failing when run on 2.4 kernels.
This can be worked around by redefining PTRACE_SETOPTIONS to
PTRACE_OLDSETOPTIONS, if that is defined.
Group-stop notifications are sent to the tracer, but not to real
parent. Last confirmed on 2.6.38.6.
If a thread group leader is traced and exits by calling _exit(2), a
PTRACE_EVENT_EXIT stop will happen for it (if requested), but the
subsequent WIFEXITED notification will not be delivered until all
other threads exit. As explained above, if one of other threads
calls execve(2), the death of the thread group leader will never be
reported. If the execed thread is not traced by this tracer, the
tracer will never know that execve(2) happened. One possible
workaround is to PTRACE_DETACH the thread group leader instead of
restarting it in this case. Last confirmed on 2.6.38.6.
A SIGKILL signal may still cause a PTRACE_EVENT_EXIT stop before
actual signal death. This may be changed in the future; SIGKILL is
meant to always immediately kill tasks even under ptrace. Last
confirmed on Linux 3.13.
Some system calls return with EINTR if a signal was sent to a tracee,
but delivery was suppressed by the tracer. (This is very typical
operation: it is usually done by debuggers on every attach, in order
to not introduce a bogus SIGSTOP). As of Linux 3.2.9, the following
system calls are affected (this list is likely incomplete):
epoll_wait(2), and read(2) from an inotify(7) file descriptor. The
usual symptom of this bug is that when you attach to a quiescent
process with the command
strace -p <process-ID>
then, instead of the usual and expected one-line output such as
restart_syscall(<... resuming interrupted call ...>_
or
select(6, [5], NULL, [5], NULL_
('_' denotes the cursor position), you observe more than one line.
For example:
clock_gettime(CLOCK_MONOTONIC, {15370, 690928118}) = 0
epoll_wait(4,_
What is not visible here is that the process was blocked in
epoll_wait(2) before strace(1) has attached to it. Attaching caused
epoll_wait(2) to return to user space with the error EINTR. In this
particular case, the program reacted to EINTR by checking the current
time, and then executing epoll_wait(2) again. (Programs which do not
expect such "stray" EINTR errors may behave in an unintended way upon
an strace(1) attach.)

This page is part of release 4.08 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2016-07-17 PTRACE(2)